Stepping motor driver

ABSTRACT

A driver circuit for controlling the excitation of windings of a multiphase stepping motor according to a sequential switching format includes a plurality of phase activation circuits each associated with a respective one of the windings. The activation circuits are coupled together in pairs and contain active elements arranged to perform in the manner of a flip-flop in response to the switching format so that first one and then the other excites its respective winding in complementary fashion, in that when one of the windings associated with the pair of activation circuits is on, the other is off, and vice versa. An electrical energy storage device is associated with and coupled to each pair of activation circuits for storing energy from the power supply and from the collapsing magnetic field of the deenergized winding, and for supplying the stored energy to the windings in the proper sequence during predetermined portions of the switching format, thereby raising the average current to the windings and the torque on the shaft.

United States Patent Newell 51 June 20, 1972 [54] STEPPING MOTOR DRIVERHarold R. Newell, South Newbury, N.H.

Mesur-Matic Electronics Corp., Warner, NH.

22 Filed: Sept.11,1969

2| Appl.No.: 857,158

[72] lnventor:

[73] Assignee:

Primary E.\'aminer-G. R. Simmons Attorney-Hurvitz, Rose & Greene [5 7]ABSTRACT A driver circuit for controlling the excitation of windings ofa multiphase stepping motor according to a sequential switching formatincludes a plurality of phase activation circuits each associated with arespective one of the windings. The activation circuits are coupledtogether in pairs and contain active elements arranged to perform in themanner of a flip-flop in response to the switching format so that firstone and then the other excites its respective winding in complementaryfashion, in that when one of the windings associated with the pair ofactivation circuitsis on, the other is off, and vice versa. Anelectrical energy storage device is associated with and coupled to eachpair of activation circuits for storing energy from the power supply andfrom the collapsing magnetic field of the deenergized winding, and forsupplying the stored energy to the windings in the proper sequenceduring predetermined portions of the switching format, thereby raisingthe average current to the windings and the torque on the shaft.

6 Claims, 2 Drawing Figures ST EPPlNG MOTOR PA'TENTEDJUH 2 0 m2 ZFIGJ.

l STEPPNG MOTOR PHQSE INVENTOR HAROLD R. NEHJELL ATTORNEYS STEPPINGMOTOR DRIVER BACKGROUND OF THE INVENTION The present invention relatesgenerally to control circuits and more particularly, to drive circuitsfor multiphase stepping motors.

In general, multiphase stepping motors are operated or driven bysequentially switching a d-c supply voltage to the several windings forequal increments of time. For example, a four-phase stepping motorhaving successive field windings A, B, C, and D arranged relative toeach other and to the rotor such as to produce upon energization, amagnetic field tending to exert a torque on the shaft coupled to therotor, would normally be stepped through discrete equiangular shaftorientations by selectively applying a driving voltage to the windings(coils) A, B, C, D, A, B, C, D, etc. for equal intervals of time.Certain improvements in the stepping motor operation, and specificallyan increase in torque, may be obtained by a variation of the sequentialenergization of windings, namely, application of the energizing voltageto two windings at a time in the equal time interval sequence, such asby exciting the windings according to the format A+B, B-l-C, C+D, D+A,A+B, etc.

An increase in switching speed to produce higher incremental rotationalvelocities of the shaft, that is, high speed stepping, however, ischaracterized by a reduction in coil current and consequently areduction in the torque-producing magnetic field, because of the motorcoil inductance. As is well-known, current through an inductive elementcannot undergo an instantaneous (non-continuous) change in magnitudeexcept by supplying the inductor with discrete packages of current inthe form of impulses of substantially infinitesimal width (i.e., impulsewidths approaching zero). It

is possible to assure sufficient current for high switching speeds byselectively increasing the supply voltage, but this method isimpractical, although certainly readily achievable, because it requiresadvance knowledge of exact times that high stepping speeds will berequired, information which is not generally foreseeable or available apriori, and because it leads to excessive winding currents, as aconsequence of the increased supply voltage, when motor operation isceased.

Another method of providing sufiicient current for the desired highswitching speed is implemented by use of a dropping resistor andcapacitor in parallel circuit through which an increased supply voltageis applied to the windings. The capacitor serves to ensure rapidbuild-up of current through the coils during switching and the droppingresistor functions to reduce winding currents during periods when themotor is stopped. It is apparent that this method, like that mentionedimmediately above, requires an abnormally high power supply voltage.Moreover, in the former case the voltage must be varied according to thedesired motor speed, while in the latter case a significant amount ofpower is dissipated in the dropping resistor.

In my application for U.S. Pat, Ser. No. 581,334, filed Sept. 22, 1966,entitled Multi-Phase Step Motor Control Circuits," now U.S. Pat. No.3,444,447, I havedisclosed multiphase driver circuitry wherein aplurality of switching circuits, corresponding in number to the numberof stepping phases to which the motor is to be subjected, aresequentially activated according to a predetermined switching logicprogram. The field windings of the motor are excited according to theenergization of the driver switching circuits with which they areassociated so that each step phase has on periods, i.e., time intervalsduring which the field winding corresponding to that phase is excited,and off periods, i.e., intervals during which there is no excitation ofthe corresponding field winding. Each switching circuit is arranged andadapted to store energy from the overall circuit power supply during theoff periods of its associated phase and to supply the stored energyalongwith the normally available energy from the circuit power supply to thefield winding for that phase during its predetermined on" period, so asto compensate for the finite current rise time owing to the inductanceof the winding. In other words, where high switching speeds are desiredthe driver provides a torque boost" to the motor by increasing the speedat which the torque-producing magnetic field of each winding builds up,and this is accomplished without the necessity of increasing the outputof the power supply. On the other hand, for low switching speedoperation the average increase in the amount of energy supplied tothewindings is practically negligible, so that, in overall effect, thedriver circuit automatically adjusts motor driving torque throughout anyvariations in switching speed which may be necessary or desirable forthe stepping motor operation.

In the prior art, the stored energy, which is released when the fieldaround the winding decays, is lost in the voltage limiting means used toprevent damage to the switching transistors. At high stepping rates,this lost energy alone can be equal to or even greater than the energyrequired to run the motor at low rates.

While the invention disclosed in my aforementioned copending applicationperforms admirably in carrying out its intended purpose, I have foundthat certain improvements in operation may be obtained by storing in anenergy storage device not only some of the energy that may be obtainedfrom the system power supply, but by additionally storing energy fromeach winding that was previously turned off, that is to say, storingenergy from the collapsing magnetic field that occurs when current to awinding is terminated.

Accordingly, it is a broad object of the present invention to provide astepping motor driver which overcomes the normal impediment to rapidrise of current in a motor winding that is newly turned on, so thatduring rapid switching of current to the field winding required duringhigh speed stepped shaft rotation, the average current to the motor, andthus the shaft torque, is maintained at a relatively high level.

SUMMARY OF THE INVENTION Briefly, according to the present invention, asingle energy storage device is associated with each pair of fieldwindings or phases of the motor, that is, the number of such devices isequal to the number of pairs of windings, and is so arranged in thedriver circuit that it responds to the activation of one of theassociated phases to store energy from the power source for the driver,and upon activation of the other associated phase, to supply the storedenergy to the first phase causing a momentary increase in the current inthe winding constituting that phase. The effect over a number of stepsis to increase the average current to the motor, and accordingly, thetorque on the shaft.

Moreover, as the magnetic field of the first phase collapses followingthe momentary current increase in its winding (at that point, the firstphase has been deactivated in so far as that can be accomplished by theabsence of a respective one of the extemalenergizing pulses supplied tothe motor driver), in response to the continued release of the storedenergy from the storage device, the storage device is now supplied withenergy from the decaying field which may amount to several times theenergy supplied by the power source for the driver. As the second phaseis deactivated and the first phase re-activated by the absence orpresence, respectively, of external energizing pulses to the motordriver in accordance with the switching logic, the energy now present inthe storage device is released to significantly increase the level ofcurrent in the second phase winding. This too has the effect of raisingthe average current to the motor windings, and thereby, increasing theavailable torque.

The process is repetitive, taking power that would otherwise be largelydissipated in the form of heat, or would be retained by the power supplyand unavailable at specific portions of the phase switching cycle, foruse in subsequently supplying higher-energy levels to the various phasesat specific portions of the switching cycle.

BRIEF DESCRIPTION OF THE DRAWINGS The above and still further objects,features and attendant advantages of the present invention will becomeapparent from a consideration of the following detailed description of apreferred embodiment thereof, especially when taken in conjunction withthe accompanying drawings in which:

FIG. 1 is a circuit diagram of a four-phase stepping motor driveraccording to my invention; and

FIG. 2 is a tabulation exemplifying the switching logic for the motordriver of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, theseveral windings of a multiphase stepping motor schematicallyrepresented by the dotted block designated 10, are connected torespective phase activation circuits or switching circuits of a motordriver circuit. The stepping motor may be of any well-known type such asthat manufactured by the United Shoe Machinery Corporation as their HDUMseries (e.g., I-IDUM-30-7), marketed under the trade name Responsyn," orthe Superior Electric Company SLO SYN" bifilar stepping motor. The motorrepresented in FIG. 1 has four phases with respective windingsdesignated 11, 12, 13, and 14.

Each of the phase activation circuits of the overall driver circuit forthe motor includes a transistor 17, 18, 19, 20, respectively. Thecollector of each transistor is coupled via series connected diodes,such as 41, 46 in the case of transistor 17, poled anode to cathode inthe direction of the respective winding of the motor phase to beactivated by that particular circuit. The several phase activatingcircuits of the overall motor driver circuit are connected in pairs andare so energized that each pair of transistors effectively operates as aflipflop. Specifically, the activating circuits for phases 1 and 3 formone pair and the activating circuits for phases 2 and 4 form the secondpair; As will become apparent from the ensuing description, the windingsassociated with the respective phases are excited in such a manner thatthey form complementary pairs in which one phase is on when the other isoff, and vice versa. If more phases were available in a specific motorconfiguration, then an appropriately greater number of activatingcircuits would of course be utilized. In the activating circuits forphases 1 and 3, the emitter electrodes of transistors 17 and 19 areconnected together and to the cathode of a diode 40 whose anode isgrounded or connected to some other point of reference potential.Energizing pulses for the various activation circuits by which toprovide the desired switching logic for the various motor phases aresupplied to temlinals such as 21, 23 connected to the base electrodes ofthe respective transistors 17, 19.

Thejunctions between the diodes by which the transistors in eachrespective pair of activation circuits are connected to the respectivephase windings are themselves connected by a capacitor, such as 30 fortransistors l7, 19, which capacitor is to be utilized as the energystorage device in the preferred embodiment. The same general arrangementand interconnection of components occurs in the activation circuit pairsfor the other pairs of phases and hence the description of one such pairis sufficient. Power is supplied to the motor driver circuit by a powersupply 75 connected in common to the other end of each of the windingsof motor 10. The power supply provides a negative voltage (for example,minus 75 volts) by which to energize the various phases in accordancewith the switching logic and to provide necessary biasing for thetransistors. The polarity of the power supply depends of course upon thetypes of components used and would obviously be required to provide apositive voltage output if NPN transistors were used in place of thePNP-type shown. In the latter case, the directions in which the diodesare poled would also be reversed from those shown.

In operation, each pair of phase activation circuits is normallyenergized to operate similarly to a flip-flop, in the sense that anenergizing pulse of appropriate polarity is first applied to terminal21, and after some predetermined time interval a second energizing pulseof appropriate polarity is applied to terminal 23 simultaneously withthe removal of the energizing pulse from terminal 21, and so forth. If,for example, transistor 17 is driven from its normal cut-off state to astate of saturation, or more generally, from a non-conductive to aconductive condition, then diodes 40, 41, and 46 are forward biased as aresult of the voltage levels existing through that portion of thecircuit and phase 1 is activated by the current flow through winding 11.At the same time, phase 3 is momentarily activated as a result of thetransient current flow through winding 13 via the forward biased diodes40, 41 and 42, and capacitor 30. Except for the instant at whichtransistor 17 is turned on, however, the energization current level inwinding 13 is far less than that in winding 11 and approaches zero ascapacitor 30 charges to the level of voltage available from power supply75. Accordingly, the effect of this very fleeting activation of phase 3that results in the charging of capacitor 30 from the power supply, isotherwise practically negligible in the motor operation.

When transistor 19 is turned on and transistor 17 is turned off, point50 is driven, disregarding some inconsequential voltage drops across thediodes and the transistor, to the level of the reference potential atthe anode of diode 40. Since capacitor 30 is charged to approximatelythe level of power supply 75, it follows that point 51 on the other sideof the capacitor assumes a voltage level equal to approximately thepower supply level above ground, the reference level at point 50. Atthis moment, then, the voltage difference between the two ends ofwinding 11 is equal to approximately twice the output voltage of powersupply 75, and there is accordingly a momentary increase in the currentthrough winding 11 (phase number 1) of the stepping motor. In effect,that phase is supplied with an impulse of current, as capacitor 30undergoes a relatively rapid discharge, at the very instant that winding11 would otherwise be deactivated as a result of the turning off oftransistor 17. Taken over a large number of steps, this operation, whichoccurs during a portion of each switching cycle, results in asubstantial increase in the average current supplied to the motorwindings, and is particularly effective during high speed switching toincrease the shaft torque over what it would ordinarily be in theabsence of an addition to average current level.

In the prior art, the stored energy, which is released when the fieldaround the winding decays, is lost in the voltage limiting means used toprevent damage to the switching transistors. At high stepping rates,this lost energy alone can be equal to or even greater than the energyrequired to run the motor at low rates.

Following this momentary increase of current through winding 11, themagnetic field associated with phase number 1 immediately begins todecay with the discharge of capacitor 30. The collapsing magnetic fieldproduces a reverse voltage surge or spike that drives point 51 to apotential level of up to approximately three times the output level ofpower supply 75. This voltage is retained by capacitor 30 because diode46 decouples winding 11 from capacitor 30 when the magnetic fieldassociated with that phase has collapsed. That is to say, the cathode ofdiode 46 is now at the level of power supply 75 while the anode of thediode is at a voltage level significantly more negative than the levelof supply 75.

When, during the next appropriate interval, transistor 19 is turned offand simultaneously therewith transistor 17 is turned back on, point 51is driven to approximately ground level, disregarding the negligiblevoltage drops across diodes 40 and 41 and transistor 17, point 50 isdriven to a positive level equal to the negative supply voltage plus thevoltage supplied by the capacitor 30 in terms of its stored energy dueto the decay spike. This obviously greatly increases the current flowthrough winding 13 associated with phase number 3, which feels theeffect of not only the power supply level, but the voltage stored bycapacitor 30. Here again, a package or impulse of current is momentarilysupplied to a phase winding at the very moment when it would otherwisebe deactivated as a result of removal of the energizing pulse at itsrespective activation circuit. That is to say, the momentary increase incurrent flow through winding 13 occurs at a time when transistor 19 isturned off and transistor 17, which activates phase number 1, is turnedon.

The operation which has thus far been described is repeti-v tive, and itwill be observed that the activation circuit and energy storage devicearrangement advantageously stores energy from the circuit power supplyduring intervals when one of the associated phase is turned on andsupplies that energy to that same phase at the moment when it wouldotherwise be turned offf; and that the storage device further receivesand retains energy from the decaying magnetic field of that phase tosupply the stored energy to the other associated phase when the latterphase would otherwise be turned off. Each actively coupled pair ofactivation circuits performs in this manner, being energized in aflipflop manner according to signals supplied to input terminals 21, 23and 22, 24

Switches 17, 18, 19 and are preferably transistors, and in that eventmust be utilized with diodes in the manner shown in FIG. 1. Diodes 42and 46, for example, are necessary to prevent the loop includingwindings 11 and 13 of the motor and capacitor 30 from undergoingoscillation in general, and damped oscillation in particular, each timethe capacitor receives a charge. Without these two diodes, then, thevoltage level available on the capacitor would be insufficient to ensurethe commutation by which the stored energy is supplied to theappropriate windings. Diodes 41 and 45 are utilized to protect theirrespective transistors'against damage upon the occurrence of a voltagereversal (e.g., reverse voltage spike) in the motor windings. Diode 40serves to provide the proper bias for the transistor which is cut 0E.That is, the current flow through diode 40, when transistor 17 is turnedon, for example, produces a voltage level at the cathode of that diodefor appropriately biasing transistor 19 back to its cut-off state.

Each of the capacitors, such as 30, is also important in limiting thevoltage level to which the collector of the off transistor can be drivenby the collapsing field of the winding, to prevent breakdown of thattransistor. In addition, each capacitor ensures that the voltage risesslowly, thereby providing sufficient time for the transistors to turncompletely off before extremely high voltage levels are applied.

Referring now to FIG. 2, an exemplary switching logic diagram for thedriver circuit of FIG. 1, the energizing pulses are applied to terminals2124 in a sequence to provide energization of two windings at a timeduring any given time interval. For example, during time interval number1, phases 1 and 2 are turned on by applying energizing pulses toterminals 21 and 22. At the conclusion of time interval number 1,energizing pulses are applied to input terminals 22 and 23 to activatephases 2 and 3. This phase activation continues in the manner shown inthe table of FIG. 2, in the direction of increasing time interval forthe arbitrarily selected forward direction of shaft rotation, and in thedecreasing direction of time interval for the opposite or reversedirection of shaft rotation.

It will be observed that for this exemplary energization of two windingsat any given interval of time, one winding remains on during twosuccessive time intervals. For example, phase 2 is on during timeintervals 1 and 2 for the switching logic diagram of FIG. 2. In such acase, the energizing voltage is maintained uniformly at terminal 22throughout intervals 1 and 2 and a complete cycle of the flip-flopaction (of transistors 18 and 20, for example) requires a period of fourtiming intervals of the switching logic.

Iclaim:

1. Control circuitry for exciting the field windings of a mu]- tiphasestepping motor, comprising a driver circuit for supplying energizingcurrents to said field windings in a predetermined sequential switchingfonnat, so that each field winding is energized during certainpreselected time increments of said switching format and de-energizedduring certain other preselected time increments of said switchingformat; a power supply for said driver circuit; said driver circuitincluding a plurality of switching circuits corresponding in number tothe number of stepping phases of said motor, said switching circuitscoupled in pairs to respective pairs of said field windings constitutingrespective phases of said motor and each of said switching circuitsincluding in series one of said windings, two series connected diodesand a phasing switch, all in the order recited, and means associatedwith each respective pair of switching circuits for storing energy fromsaid power supply during energization of one of the associated fieldwindings and for supplying the stored energy in the form of additionalenergizing current to said one as sociated field winding at theconclusion of its normal energization interval, and for storing energyfrom the collapsing magnetic field of said one associated field windingupon de-energization thereof and for supplying the last-named storedenergy in the form of additional energizing current to the otherassociated field windings at the conclusion of its normal energizationinterval, said storing and supplying means comprising a capacitorinterconnecting each pair of said switching circuits, and wherein eachof said switching circuits includes one of said phasing switchesresponsive to said switching format to open and close circuitscontaining respective field windings through said capacitor to producesaid storing and supplying operations during said intervals. 2. Controlcircuitry for exciting the field windings of a mu]- tiphase steppingmotor, comprising a driver circuit for supplying energizing currents tosaid field windings in a predetermined sequential switching format, sothat each field winding is energized during certain preselected timeincrements of said switching format and de-energized during certainother preselected time increments of said switching format; a powersupply for said driver circuit; said driver circuit including aplurality of switching circuits corresponding in number to the number ofstepping phases of said motor, said switching circuits coupled in pairsto respective pairs of said field windings constituting respectivephases of said motor, and means associated with each respective pair ofswitching circuits for storing energy from said power supply duringenergization of one of the associated field windings and for supplyingthe stored energy in the form of additional energizing current to saidone associated field winding at the conclusion of its normalenergization interval, and for storing energy from the collapsingmagnetic field of said one associated field winding upon de-enerizationthereof and for supplying the last-named stored energy in the form ofadditional energizing current to the other associated field windings atthe conclusion of its normal energization interval, said means forstoring and supplying comprising a capacitor interconnecting each pairof said switching circuits, and wherein each of said switching circuitsincludes switch means responsive to said switching format to open andclose circuits containing the respective field windings through saidcapacitor to produce said storing and supplying operations during saidintervals, and switch means comprising a transistor for each switchingcircuit, and wherein is further provided in each switching circuit firstdiode means for protecting said transistor from reverse voltage surgesin said windings and second diode means to prevent oscillation of acompleted circuit containing said capacitor and associated fieldwindings.

3. The invention according to claim 2 wherein the controllable currentpath of said transistor is connected in series circuit with said firstdiode means, said second diode means, the respective field winding, andsaid power supply in each switching circuit; and wherein said capacitoris connected from the junction of said first and said second diode meansin one switching circuit to the junction of said first and said seconddiode means in the other switching circuit for a respective pair of saidswitching circuits; and wherein the end of the controllable current pathof the transistor opposite that to which said first diode means isconnected is coupled to the corresponding end of the transistor in theother switching circuit of that pair and to a point of referencepotential.

4. Apparatus for controlling the excitation of windings of a multiphasestepping motor for equal intervals of time according to a predetemiinedsequential excitation format, said format characterized by theexcitation of certain windings during intervals in which certain otherwindings are unexcited and vice versa, so that the windings formcomplementary pairs; said apparatus comprising a plurality ofnon-inducive phase activation circuits, each associated with arespective one of said windings;

a d-c power supply coupled to all of said activation circuits;

said activation circuits coupled together in pairs and each containingin series one of said windings, two series connected diodes and aphasing switch all in the order recited, said phasing switches includingactive transistor elements arranged to undergo flip-flop operation inresponse to said excitation format, so that first the activation circuitassociated with one winding and then the activation circuit associatedwith the other winding of a complementary pair supply energizing currentto the respective winding from said d-c power supply; and

one electrical energy storage means connected between each pair ofactivation circuits for storing electrical ener gy from said powersupply and from the collapsing magnetic field of the de-energizedwinding during portions of the excitation format and for supplyingstored energy to the windings in the proper sequence during otherportions of the excitation format to increase the average current tosaid windings over a substantial number of steps of the stepping motorrelative to the value of average current to said windings in the absenceof energy storage.

5. A driver circuit for the windings of a step motor having a pluralityof complementary stepping phases, comprising a separate transistorswitching circuit for each phase, connected to the respective windingassociated with that phase, and each said transistor switching circuitincluding a transistor phasing switch and two diodes connected in seriesbetween said respective winding and said phasing switch, each saidswitching circuit effective, when energized, to excite said respectivewinding for a predetermined interval of time,

one separate capacitive electrical energy storing means connectedbetween each pair of switching circuits connected to a pair ofcomplementary windings, and

means connecting the switching circuits associated with a pair ofcomplementary windings together via the respective electrical energystoring means associated with those switching circuits such that uponenergization in alternation of each said pair of switching circuits, theelectrical energy storing means connected thereto stores energy from thecollapsing magnetic field of the winding whose excitation has beenremoved and supplies the stored energy to the complementary winding inor contiguous with its next interval of excitation.

6. In a multi-phase step motor having windings of phases 1 to n, where nis an even number, means for energizing said phases alwayssimultaneously in adjacent pairs and sequentially selecting the pairstimewise to accomplish a complete sequential scan of all the phaseswhile only maintaining adjacent pairs energized,

a terminal connected between a voltage source and a reference point,parallel circuits extending between said terminal and said referencepoint,

each of said parallel circuits including in series one of said windings,two series connected diodes poled to be conductive of current betweensaid terminal and said reference point, and a phasing switch, all in theorder recited, and

a separate storage capacitor connected between the junctions of theseries connected diodes of all pairs of said windings numbered bynumbers separated by unity.

1. Control circuitry for exciting the field windings of a multiphasestepping motor, comprising a driver circuit for supplying energizingcurrents to said field windings in a predetermined sequential switchingformat, so that each field winding is energized during certainpreselected time increments of said switching format and de-energizedduring certain other preselected time increments of said switchingformat; a power supply for said driver circuit; said driver circuitincluding a plurality of switching circuits corresponding in number tothe number of stepping phases of said motor, said switching circuitscoupled in pairs to respective pairs of said field windings constitutingrespective phases of said motor and each of said switching circuitsincluding in series one of said windings, two series connected diodesand a phasing switch, all in the order recited, and means associatedwith each respective pair of switching circuits for storing energy fromsaid power supply during energization of one of the associated fieldwindings and for supplying the stored energy in the form of additionalenergizing current to said one associated field winding at theconclusion of its normal energization interval, and for storing energyfrom the collapsing magnetic field of said one associated field windingupon de-energization thereof and for supplying the last-named storedenergy in the form of additional energizing current to the otherassociated field windings at the conclusion of its normal energizationinterval, said storing and supplying means comprising a capacitorinterconnecting each pair of said switching circuits, and wherein eachof said switching circuits includes one of said phasing switchesresponsive to said switching format to open and close circuitscontaining respective field windings through said capacitor to producesaid storing and supplying operations during said intervals.
 2. Controlcircuitry for exciting the field windings of a multiphase steppingmotor, comprising a driver circuit for supplying energizing currents tosaid field windings in a predetermined sequential switching format, sothat each field winding is energized during certain preselected timeincrements of said switching format and de-energized during certainother preselected time increments of said switching format; a powersupply for said driver circuit; said driver circuit including aplurality of switching circuits corresponding in number to the number ofstepping phases of said motor, said switching circuits coupled in pairsto respective pairs of said field windings constituting respectivephases of said motor, and means associated with each respective pair ofswitching circuits for storing energy from said power supply duringenergization of one of the associated field windings and for supplyingthe stored energy in the form of additional energizing current to saidone associated field winding at the conclusion of its normalenergization interval, and for storing energy from the collapsingmagnetic field of said one associated field winding upon de-enerizationthereof and for supplying the last-named stored energy in the form ofadditional energizing current to the other associated field windings atthe conclusion of its normal energization interval, said means forstoring and supplying comprising a capacitor interconnecting each pairof said switching circuits, and wherein each of said switching circuitsincludes switch means responsive to said switching format to open andclose circuits containing the respective field windings through saidcapacitor to produce said storing and supplying operations during saidintervals, and switch means comprising a transistor for each switchingcircuit, and wherein is further provided in each switching circuit firstdiode means for protecting said transistor from reverse voltage surgesin said windings and second diode means to prevent oscillation of acompleted circuit containing said capacitor and associated fieldwindings.
 3. The invention according to claim 2 wherein the controllablecurrent path of said transistor is connected in series circuit with saidfirst diode means, said second diode means, the respective fieldwinding, and said power supply in each switching circuit; and whereinsaid capacitor is connected from the junction of said first and saidsecond diode means in one switching circuit to the junction of saidfirst and said second diode means in the other switching circuit for arespective pair of said switching circuits; and wherein the end of thecontrollable current path of the transistor opposite that to which saidfirst diode means is connected is coupled to the corresponding end ofthe transistor in the other switching circuit of that pair and to apoint of reference potential.
 4. Apparatus for controlling theexcitation of windings of a multiphase stepping motor for equalintervals of time according to a predetermined sequential excitationformat, said format characterized by the excitation of certain windingsduring intervals in which certain other windings are unexcited and viceversa, so that the windings form complementary pairs; said apparatuscomprising a plurality of non-inducive phase activation circuits, eachassociated with a respective one of said windings; a d-c power supplycoupled to all of said activation circuits; said activation circuitscoupled together in pairs and each containing in series one of saidwindings, two series connected diodes and a phasing switch all in theorder recited, said phasing switches including active transistorelements arranged to undergo flip-flop operaTion in response to saidexcitation format, so that first the activation circuit associated withone winding and then the activation circuit associated with the otherwinding of a complementary pair supply energizing current to therespective winding from said d-c power supply; and one electrical energystorage means connected between each pair of activation circuits forstoring electrical energy from said power supply and from the collapsingmagnetic field of the de-energized winding during portions of theexcitation format and for supplying stored energy to the windings in theproper sequence during other portions of the excitation format toincrease the average current to said windings over a substantial numberof steps of the stepping motor relative to the value of average currentto said windings in the absence of energy storage.
 5. A driver circuitfor the windings of a step motor having a plurality of complementarystepping phases, comprising a separate transistor switching circuit foreach phase, connected to the respective winding associated with thatphase, and each said transistor switching circuit including a transistorphasing switch and two diodes connected in series between saidrespective winding and said phasing switch, each said switching circuiteffective, when energized, to excite said respective winding for apredetermined interval of time, one separate capacitive electricalenergy storing means connected between each pair of switching circuitsconnected to a pair of complementary windings, and means connecting theswitching circuits associated with a pair of complementary windingstogether via the respective electrical energy storing means associatedwith those switching circuits such that upon energization in alternationof each said pair of switching circuits, the electrical energy storingmeans connected thereto stores energy from the collapsing magnetic fieldof the winding whose excitation has been removed and supplies the storedenergy to the complementary winding in or contiguous with its nextinterval of excitation.
 6. In a multi-phase step motor having windingsof phases 1 to n, where n is an even number, means for energizing saidphases always simultaneously in adjacent pairs and sequentiallyselecting the pairs timewise to accomplish a complete sequential scan ofall the phases while only maintaining adjacent pairs energized, aterminal connected between a voltage source and a reference point,parallel circuits extending between said terminal and said referencepoint, each of said parallel circuits including in series one of saidwindings, two series connected diodes poled to be conductive of currentbetween said terminal and said reference point, and a phasing switch,all in the order recited, and a separate storage capacitor connectedbetween the junctions of the series connected diodes of all pairs ofsaid windings numbered by numbers separated by unity.